Will Saletan writes about politics, science, technology, and other stuff for Slate. He’s the author of Bearing Right.

Three weeks ago, the U.S. armed forces tested it on volunteers at an Air Force base in Georgia. You can watch the video on a military Web site. Three colonels get zapped, along with an Associated Press reporter. The beam is invisible, but its effects are vivid. Two dozen airmen scatter. The AP guy shrieks and bolts out of the target zone. He says it felt like heat all over his body, as though his jacket were on fire.

The feeling is an illusion. No one is harmed. The beam's energy waves penetrate just one-sixty-fourth of an inch into your body, heating your skin like microwaves. They inflame your nerve endings without actually burning you. This could be the future of warfare: less bloodshed, more pain.

Military technology has always sought greater precision from longer range. In the Gulf War, Kosovo, and Afghanistan, we exploited the increasing accuracy of laser-guided bombs. In the post-9/11 terrorist hunt and the occupation of Iraq, we've sent hundreds of remotely piloted aerial drones to spy and kill. But the lives protected by drones are ours. The pain beam is more ambitious: It can spare civilians and even the enemy. Precision isn't just the ability to kill. Sometimes, it's ability to disperse and deter without killing.

That kind of precision is becoming more important. Twelve years ago, the Department of Defense observed that our armed forces were increasingly being used for peacekeeping, humanitarian aid, and protection of civil society. More of our enemies were blending in with, or disguising themselves as, civilians. Through the media, more eyeballs, hearts, and minds could see the infrastructure we destroyed. The DOD proposed the development of weapons "to incapacitate personnel or materiel, while minimizing fatalities, permanent injury to personnel, and undesired damage to property and the environment."

Like lethal weapons, nonlethal weapons have evolved from short- to long-range. Batons and pepper spray required hand-to-hand combat. Water cannons, rubber bullets, beanbag rounds, and sting ball grenades have extended our reach, but not far enough to keep soldiers clear of rocks or small-arms fire. Some of our weapons are insufficiently discriminate. Tear gas torments a whole crowd, not just the miscreants using it for cover.

Projectiles are also unpredictable. At long range, particularly in crosswinds, rubber bullets can hit the wrong people, or the right people in the wrong places. At close range, they can kill. Look at the absurdly named "FN303 less lethal launcher." It's supposed to fire "non-lethal projectiles at established non-lethal ranges." But when you're launching things, less lethal is the best you can do.

That's where the pain beam comes in. Unlike projectiles, beams are "directed energy." They travel in a straight line over long distances, ignoring gravity and wind. They cause no more damage at 10 feet than at 1,000. Unlike gas, they're discriminate. Raytheon, the pain beam's manufacturer, points out that the weapon "allows precise targeting of specific individuals" and that the pain "ceases immediately" when the beam is diverted or the target flees.

The shift from hardware to software, from matter to energy, can do more than control the unpredictability of weapons. It can control the unpredictability of the people who fire them. Early nonlethal devices, such as rubber bullets and Mace, often caused injuries due to abuse by hotheads. When the pain beam was initially being developed, somebody accidentally fired it on a high setting, inflicting a second-degree burn. The designers responded by programming limits on the beam's power and duration.

Years of work have gone into making the beam safe. It's been tested thousands of times on 600 volunteers. It's been reviewed and revised by a human-effects review board, a human-effects advisory panel, and military surgeons general. It's been tested for effects on skin cancer, fertility, jewelry, and drunks. The results have been published in peer-reviewed journals. Never has an organization licensed to kill jumped through so many hoops to make sure nobody gets injured.

But the ability to inflict pain without injury doesn't just make injury less necessary. It makes pain more essential to military operations—and easier to inflict. To achieve the desired "repel effect," I have to make you suffer. Knowing that your agony will be brief and leave no physical damage makes the weapon easier to fire. It's almost as though, like the imagined flames on the AP reporter's jacket, your pain isn't real.

That's the metaphysical gap nonlethal energy weapons exploit. The rain of pain falls mainly in the brain. The long-range acoustic device, for instance, "can target narrow sound beams at excruciating decibel levels, but below the threshold of hearing damage," according to a military account of a presentation by its project manager. Four months ago, Congresspassed and President Bush signed legislation to prosecute torture, defined as intentional infliction of "serious physical or mental pain or suffering." But that rule applies only in captivity. On the street, pain administration won't be a crime. It'll be a policy.

Two weeks from now, military leaders will convene in London to discuss the pain beam and the next generation of directed-energy weapons, including microwaves and lasers. Law enforcement agencies are interested. Raytheon is already advertising the technology for commercial applications. We're even developing a "personnel halting and stimulation response" system—yes, a PHaSR—to stun targets instead of killing them. But don't worry, nobody will get hurt. Sort of.

A version of this article also appears in the Outlook section of the Sunday Washington Post.